"Perhaps even that is possible," said Tarzan, who was now evincing increasing interest in Gridley's proposition.
The American shook his head. "It may be possible some day," he said, "but not at present with any known material. Any receptacle having sufficient strength to withstand the atmospheric pressure upon a vacuum would have a weight far too great for the vacuum to lift."
"Perhaps," said Tarzan, "and, again, perhaps not."
"What do you mean?" inquired Gridley.
"What you have just said," replied Tarzan, "reminds me of something that a young friend of mine recently told me. Erich von Harben is something of a scientist and explorer himself, and the last time that I saw him he had just returned from a second expedition into the Wiramwazi Mountains, where he told me that he had discovered a lake-dwelling tribe using canoes made of a metal that was apparently as light as cork and stronger than steel. He brought some samples of the metal back with him, and at the time I last saw him he was conducting some experiments in a little laboratory he has rigged up at his father's mission."
"Where is this man?" demanded Gridley.
"Dr. von Harben's mission is in the Urambi country," replied the ape-man, "about four marches west of where we now are."
Far into the night the two men discussed plans for the project, for Tarzan was now thoroughly interested, and the next day they turned back toward the Urambi country and von Harben's mission, where they arrived on the fourth day and were greeted by Dr. von Harben and his son, Erich, as well as by the latter's wife, the beautiful Favonia of Castrum Mare.
It is not my intention to weary you with a recital of the details of the organization and equipment of the Pellucidarian expedition, although that portion of it which relates to the search for and discovery of the native mine containing the remarkable metal now known as Harbenite, filled as it was with adventure and excitement, is well worth a volume by itself.
While Tarzan and Erich von Harben were locating the mine and transporting the metal to the seacoast, Jason Gridley was in Friedrichshafen in consultation with the engineers of the company he had chosen to construct the specially designed airship in which the attempt was to be made to reach the inner world.